1. Field of the Invention
The present invention relates to a configuration of an optical switching device used in an optical access network. The present invention relates to a technique that eliminates a 1×2 splitter, which has been needed in an optical switching device, and reduces insertion loss of the optical switching device. Further, the present invention relates to a technique that compensates the loss between an optical switching device (OSM (Optical Switching Module)) and a center device (OLT (Optical Line Unit)), and extends the transmission distance between the center device and a remote device (ONU (Optical Network Unit)). The present invention also relates to a technique that achieves with extremely high accuracy the delay, which is required for downlink switching, at an electrical level instead of the conventional optical level.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 7-177098 (patent document 1) discloses a technique related to an optical access network configured into a tree-shape with one center device (OLT), a plurality of remote devices (ONU), and one optical switch connected between the OLT and an ONU.
In patent document 1, a time slot with a fixed length acts as a unit of switching. The ports are periodically connected in the downlink direction. In the uplink direction, transmission is performed after providing a delay time so that all the ONUs have a maximum delay time and the ports are periodically connected.
IEEE802.3ah™/D.3.3, “Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and physical layer specifications,” Sep. 7, 2004 (non-patent document 1) discloses a technique related to an optical access network forming a tree-shape with one center device (OLT), a plurality of remote devices (ONU), and at least one optical splitter connected between the OLT and the ONU.
Generally, such optical network is referred to as PON (Passive Optical Network), and in particular, the PON described in non-patent document 1 is called E-PON since Ethernet (registered trademark) frame is used, or GE-POM since the speed on the transmission path is gigabits.
An optical access network forming a tree-shape with one center device (OLT), a plurality of remote devices (ONU), and at least one optical switching device (OSM) connected between the OLT and the ONU is disclosed in Hiromi Ueda, Takumi Nomura, Kunitetsu Makino, Yoshinori Tsuboi, Hiroaki Kurokawa, and Hiroyuki Kasai, “Proposed New Optical Access Network Architecture-Access Networks with Optical Packet Switching”, IEICE technical report CS2004-253 (2005-03) (non-patent document 2) and Takumi Nomura, Chikashi Itoh, Hiroaki Kurokawa, Hiromi Ueda, Toshinori Tsuboi, and Hiroyuki Kasai, “Architecture of optical switching module in new optical access network”, IEICE technical report CS2004-254 (2005-03) (non-patent document 3).
A conventional OSM described in non-patent documents 2 and 3 is shown in FIG. 1. The OSM 31 includes a downlink optical switch element 10 having one input port and n output ports, and an uplink switch element 11 having n input ports and one output port. The input port of the downlink optical switch element 10 and one output port of the optical switch element 11 become one port of the OSM 31 by wavelength multiplexing, and are connected to the OLT through one optical fiber. Furthermore, output port k (k=1, 2, 3, . . . , n) of the element 10 and input port k (k=1, 2, 3, . . . , n) of the element 11 become port k (k=1, 2, 3, . . . , n) of the OSM 31 by wavelength multiplexing, and is connected to an ONU through one optical fiber.
The control of switching of the element 10 and the element 11 of the OSM 31 is performed with a signal that is divided by the 1×2 splitter (an optical splitter 60) and converted to an electrical signal. The 1×2 splitter is disposed before the input port of the element 10 and divides an electrical signal from the center device. The other optical signal is input to the element 10. The 1×2 splitter (optical splitter 60) is also arranged next to the output port of the element 11 so that packets can be transmitted from the OSM 31 to the OLT.
For more detail, the switching of the downlink optical switch element 10 of the OSM 31 is performed with an LLID (Logical Link Identifier), which is the identification number of an ONU, and the packet length. The LLID and the packet length are included in a packet obtained from the electrical signal that is converted from the optical signal of the center device (OLT). The switching of the uplink optical switch element 11 is performed with an LLID of the ONU of the destination, the transmission start time and the transmission duration of the ONU. The LLID, the transmission start time and the transmission duration are included in the GATE message obtained from the electrical signal converted from the optical signal of the center device (OLT). An output port of the element 10 and an input port of the element 11 (the port selection of the OSM 31 on the ONU side) are selected based on the LLID. As described above, the 1×2 splitters are arranged both in the downlink direction and in the uplink direction.
Among the content of non-patent document 1, the packet configuration, the transmission control of an OLT over an ONU, and the discovery operation of the OLT over the ONU will be described below. The term “packet” is consistently used herein but the content of explanation will not change even if the term “frame” is used.
The packet configuration is shown in FIG. 2. A packet mainly includes a preamble section, a MAC (Media Access Control) header section, a payload section, and an error detecting section FCS (Frame Check Sequence).
The preamble section includes a code 0×55 (01010101) for achieving bit synchronization, an LLID corresponding to the identification number of an ONU, a code 0×d5 (11010101) called SLD (Start of LLID Delimiter) for detecting an LLID, and a CRC (Cyclic Redundancy Check) for detecting bit error of the SLD and the LLID.
The MAC header section includes a destination MAC address (DA: Destination Address), a source MAC address (SA: Source Address), and length/type (L/T).
The payload section contains data of a user and data for the control of the network. There are defined five types of packets for the control of the network: namely, GATE message, REGISTER_REQ message, REGISTER message, REGISTER_ACK message and REPORT message. A time stamp is defined commonly for these messages.
The GATE message is used in the transmission control for an ONU. In the payload section of the GATE message, information such as an identification number (Opcode) of the GATE message, the time information (Time Stamp)) for distributing the time of the OLT, a discovery flag indicating whether the packet is for a discovery operation, a transmission start time (Grant Start Time) of the ONU, a transmission duration (Grant Length) of the ONU and so on are written.
The discovery operation is that an OLT provides an LLID to an ONU when a new ONU is connected or when the power of the ONU is turned on after the power is once turned off, and then a round-trip time between the ONU and the OLT is measured for the first time. The discovery operation is periodically performed to enable the provision of the LLID and the measurement of the round-trip time even if a new ONU is connected or the power of the ONU is turned off and then again turned on. The interval is determined by a system designer.
The discovery operation is shown in FIG. 3. The GATE message is transmitted from the OLT at the beginning of the discovery operation. This GATE message targets the ONU to which LLID is not given, where the LLID used therefor is that defined for broadcasting. Furthermore, the discovery flag is set to “1” and the multicast is used for the destination MAC address. Such GATE message is hereinafter referred to as “discovery GATE message”.
In the PON (Passive Optical Network), the discovery GATE message transmitted from the OLT is branched by an optical splitter, and reaches all ONUs connected to the splitter. When unregistered ONUs that are not yet given an LLID receive the discovery GATE message, they all at once transmit REGISTER_REQ message to request for registration to the OLT. In order to avoid the REGISTER_REQ messages from colliding in the interval between the optical splitter and the OLT, each unregistered ONU waits for a random time starting from the transmissions start time td2 written on the discovery GATE message, and then transmits the REGISTER_REQ message having the destination MAC address be the MAC address of the ONU.
When the OLT receives an REGISTER_REQ message, the OLT acquires the MAC address of the ONU from the REGISTER_REQ message, newly assigns an LLID, and manages the relationship between the MAC address of the ONU and the LLID. The OLT transmits the REGISTER message with the LLID written in the information region (payload region) of the packet to notify the ONU of the LLID assigned to the ONU. The ONU receives the message and obtains the LLID, and thereafter, the ONU transmits packets with the LLID given to the preamble section of a packet. The ONU also determines whether the packet sent from the OLT is for itself based on the LLID in the preamble section. If the LLID in the preamble section and the LLID written in the data region of the REGISTER message must be specifically distinguished, the latter will be described as LLID_Reg.
Subsequently, the OLT specifies the ONU with the LLID, and the GATE message with the transmitting MAC address being “multicast” and the discovery flag being 0 is transmitted to measure the round-trip time (called ranging). Such GATE message is hereinafter referred to as “ranging GATE message”. After receiving the ranging GATE message, the ONU corresponding to the LLID acquires the time information (Time Stamp) tr1, the transmitting start time (Grant Start Time) tr2, and the transmitting duration (Grant Length) Tr2 written on the ranging GATE message, sets the time information tr1 for the clock of the ONU, and starts transmitting the REGISTER_ACK message at the transmitting start time tr2 of the clock to the OLT and continues it for the transmitting duration Tr2. It should be noted that tr2, written on the time information (Time Stamp) of the REGISTER_ACK message, is defined by the clock of the ONU. If the OLT receives the REGISTER_ACK message at time tr3 at its clock, the round-trip time RTTa between the OLT and the ONU can be obtained from tr2 written on the relevant message with RTTa=tr3−tr2. The measurement of the round-trip time is performed by the OLT and the registration of the ONU is completed.
In order to perform the transmission control of the ONU whose registration is completed, the OLT gives the corresponding LLID to the ONU, and uses the GATE message with the transmitting MAC address being the MAC address of the ONU and the discovery flag being 0. Such GATE message is hereinafter referred to as “transmission control GATE message”. The OLT investigates the transmission request of the ONU based on the REPORT message requested by the transmission control GATE message. Simultaneously, the OLT measures the round-trip time RTTa=t3−t2 with the transmission start time t2 written on the transmission control GATE message and the arrival time t3 of the REPORT message, and updates the measured time.
However, the above-described conventional examples have the following problems.
In the optical access network configured into a tree-shape with one center device (OLT), a plurality of remote devices (ONU), and at least one optical switching device (OSM) connected between the OLT and an ONU, the transmission distance between the OLT and the ONU is determined by the insertion loss of the OSM.
However, the 2×1 optical splitters (an optical splitter 60) are used in the downlink and uplink direction in the conventional OSM as shown in FIG. 1. The insertion loss of the 2×1 optical splitter is about 4 dB. This is added to the insertion loss of the OSM 31, and thus the insertion loss cannot be made lower than 4 dB in the entire OSM 31 even if the insertion loss of the elements 10 and 11 is reduced. Since the loss of the 1310 nm band single mode fiber used in the optical access network is about 0.34 dB/km, 4 dB is equivalent to 11.8 km. If the 2×1 optical splitter is removed from the OSM 31, the transmission distance between the OLT and the ONU can be extended by 11.8 km.
Furthermore, the delay section 51 of FIG. 1 can be realized by adjusting the optical level, for example, the length of the optical fiber, but the delay section 51 requires adjustment of nanosecond order, which is not always easy.